This doctoral thesis examines the works of Theodore Abū Qurra (d.c.829), Abū Rā’iṭa (d. c.835), and ‘Ammār al-Baṣrī (d.c.850); three of the earliest known Christian theologians to explain and defend their beliefs in Arabic, under Islamic rule. In particular, it focuses on their respective explanations of the doctrine of the Trinity: assessing each individual author’s writings; investigating the tools and arguments they employ; and considering the extent to which they engaged with Islamic theological thought (kalām), primarily through their borrowing of concepts and structures from an internal Islamic debate concerning the divine attributes of God. This study asks to what extent these Christians were essentially translating their traditional doctrine into Arabic, and to what extent they developed a new expression of the Trinity, conceptually influenced by Islamic thinking. The key conclusion of this thesis is that Christian explanations of the Trinity in Arabic, whilst they show a deep awareness of Islamic thought and make use of contemporary Muslim debates surrounding the nature and unity of God, cannot be said to represent a development in Christian theology. Rather, such works should be viewed as an informed and creative response to the pressures and challenges of their Islamic surroundings.